This invention relates to wireless memory devices, commonly known as radio frequency identification (xe2x80x9cRFIDxe2x80x9d) tags.
RFID tags are wireless communication memory devices that store information, typically concerning an item to which the RFID tag is attached. For example, inventory items can carry RFID tags providing information such as serial numbers, price, weight, and size. The RFID tags permit efficient retrieval of information regarding an item at various points in the manufacturing and distribution chain, and can permit tracking of the item. RFID tags permit relatively large amounts of data to be associated with the item. An RFID tag typically includes a memory, an RF transmitter, an RF receiver, an antenna, and logic for controlling the various components of the memory device. The antenna is generally formed on a flexible substrate, while analog RF circuits and digital logic and memory circuits take the form of an integrated circuit (xe2x80x9cICxe2x80x9d) carried by the substrate and coupled to the antenna. RFID tags may also include a number of discrete components, such as capacitors, transistors, and diodes.
RFID tags can be either passive or active devices. Active devices are self powered, by a battery, for example. Passive devices do not contain a discrete power source, but derive their energy from the RF signal used to interrogate the RFID tag. Passive RFID tags usually include an analog circuit, which detects and decodes the interrogating RF signal and which provides power from the RF field to a digital circuit in the tag. The digital circuit generally executes all of the functions performed by the RFID tag, such as retrieving stored data from memory and modulating the RF signal to transmit the retrieved data. In addition to retrieving and transmitting data previously stored in the memory, the RFID tag can permit new or additional information to be stored into the RFID tags memory, or can be permit the RF tag to manipulate data or perform some additional functions.
A number of factors may affect the performance of an RFID tag. For example, two RFID tags may each produce different response signals in response to the same interrogation signal due to manufacturing inconsistencies. Additionally, a single RFID tag may produce different response signals in response to two different interrogation signals. For example, an RFID tag may produce a weaker response signal in response to a relatively weaker interrogation signal. This is particularly a problem with passive RFID tags, where the RFID tag derives its operating power from the interrogation signal. The strength of the interrogation signal normally varies with the inverse of the distance. Thus, an RFID tag read at two different distances will receive interrogation signals having two different power levels with which to respond.
Manufacturers and/or users typically desire a uniform response from RFID tags. For example, users will typically desire that all RFID tags in a set have a consistent output signal. Yet a user typically wants a uniform response from any RFID tag in an RFID tag reader""s range (e.g., RFID tag response signal""s strength and frequency does not vary with direction, frequency, or strength of the interrogation signal). The manufacturer and/or end user may also wish to identify poorly performing tags for removal, or to identify an appropriate frequency range for a particular set of RFID tags. Additionally, the manufacturer and/or end user may wish to label RFID tags with their respective operating characteristic values.
Under one aspect of the invention, an RFID tag verifier includes an RF interrogator that transmits first and second interrogation signals, each having a first operational characteristic that differs from the other by a known amount. The RF interrogator receives first and second return signals in response to the first and second interrogation signals, respectively. A processor is configured to determine a response of the RFID tag as defined by a second operational characteristic of each of the first and second return signals.
In one aspect, the RFID tag verifier determines the signal strength of the return signal for varying strengths of the RF interrogation signal. Typically, a flat response is desired, (i.e., relatively little change in return signal strength even where the interrogation signal strength varies significantly.
In another aspect, the RFID tag verifier determines the response in terms of signal strength of the response signals for interrogation signals having different frequencies. In some applications, a flat response will be desirable, while in other applications frequency selectivity may be desirable.
In another embodiment, the verifier determines the response in terms of frequency for interrogation signals having varying strengths.
In another aspect a machine-readable symbol verifier that verifies machine-readable symbols such as barcodes, area and stacked codes, can be coupled to, or formed as an integral part of the RFID tag verifier. Such a system allows automatic verification of RFID tags that carry machine-readable symbols. The RFID tag verifier can produce a letter grade corresponding to the response of the RFID tag and/or the quality of a machine-readable symbol.
In a further aspect, a printer can be coupled to, or formed as an integral part of, the RFID tag verifier. The printer can print information corresponding to the response of the RFID tag. The printer can, for example, print a letter grade, and/or a rejected or accepted indicia. Additionally, or alternatively, the printer can print values corresponding to an RFID tag""s response characteristics. The printer can print a report on print media, or can print directly onto the RFID tag. Printing directly on the RFID tag allows a tag""s operational characteristics to be easily ascertained. The information can be printed as human readable text, or as machine-readable symbols. Thus, the quality of RFID tags can be ensured, and RFID tags may be classed for appropriate use.